1. Field of the Invention
The present invention relates to an image-forming method using electrophotography or electrostatic recording, which has a process in which recovered toner is reused.
2. Related Background Art
Image-forming methods have been known in which an electrophotographic system or an electrostatic recording system is utilized. Various methods are disclosed, for example, in U.S. Pat. 2,297,691, Japanese Patent Publication Nos. 42-23910 and 43-24748. Generally in these methods, an electrostatic image is formed on a photosensitive member (electrostatic image holding member) constituted of a photoconductive material, the formed electrostatic image is developed with a toner, the developed toner image is transferred onto a recording medium like a paper sheet, and the transferred toner image is fixed by heating, pressing, heat-pressing, or solvent-vapor treatment.
In recent years, on laser beam printers (LBP) and copying machines employing the electrophotographic system, various requirements are imposed, such as digitization and toner particle size reduction for the purpose of realizing higher-speed printing and higher quality images, employment of an on-demand fixing system for energy saving, and reuse of waste toner (recovered toner) to meet environmental problems.
However, in meeting these requirements, various disadvantages are caused. For example, finer toner has a larger surface area per unit weight, having broader distribution of electric charge to render the toner chargeability sensitive to environmental variations. In particular, a finer particulate toner, when stored for a long period of time under high temperature and high humidity, tends to be affected by moisture to have a lower charging capacity, resulting in a lower developed image density, and toner scattering. On the other hand, under low humidity conditions, the finer toner tends to be charged excessively to cause fogging, image density drop, and sleeve ghost.
Digital copying machines are required to be capable of reproducing a letter-containing photographic image with sharpness of the reproduced letters and precise density gradation of the photograph. Generally, in reproducing a letter-containing photographic image, increase of line density for sharpness of reproduced letters impairs the density gradation of the photographic image and roughens the half-tone portion of the image. On the other hand, increase of density gradation of the image lowers the line density to impair the sharpness of the image.
In recent years, the density gradation of the copied image has become improved to some extent by digitization of the image density signal. However, further improvement is demanded. The image density is not in linear relation with the development potential (difference in the potentials between the photosensitive member and the developer holder): the curve is convexed downward at the lower development potential portion, and the curve is convexed upward at the higher development potential portion owing mainly to the characteristics of the developing agent. Therefore, at the half tone portion, slight variation in the development potential greatly changes the image density to render the density gradation unsatisfactory.
Reproduction of a line image is usually affected by the edge effect. Therefore, in the line image reproduction, the maximum density of 1.30 is sufficient at a solid image area which is less liable to be affected by the edge effect in order to keep the sharpness of the line image. On the other hand, reproduction density of a photographic image is affected greatly by surface gloss of the photograph itself, and the maximum image density is as high as 1.90 to 2.00. In the photograph image reproduction, even if the surface gloss of the photograph is reduced, the improvement of the density by the edge effect is not achievable because of the large area of the image. Therefore, in the photograph image reproduction, the maximum density ranging from 1.4 to 1.5 is necessary at a solid image area. Accordingly, it is very important to keep the maximum image density in the range from 1.4 to 1.5 for reproduction of a letter-containing photograph.
Furthermore, in the digital copying machine employing a reversal development system, the toner is moved by an electric field to a non-charged region or to a region of the same polarity and retained on the surface of the photosensitive member by the electric field generated by electrostatic induction of the toner. Therefore, in order for the toner to be transmitted while securely held on the photosensitive member, the toner chargeability should be increased so as to cause the electrostatic induction.
When the toner image is transferred, a recording medium (paper, etc.) for receiving the transferred toner image is charged electrically to the polarity opposite to that of the photosensitive member. The higher intensity of current for the transfer tends to cause problems such as winding of the recording medium by the photosensitive member by electric attraction, and re-transfer of the transferred toner to the photosensitive member. Therefore, the transfer current intensity is inevitably limited, and the electric charge of the toner should be increased to raise the releasability of the toner from the photosensitive member so as not to lower the transfer efficiency even in a weak electric field.
In a high-speed copying machine in which the photosensitive drum or photosensitive belt is rotated at a higher speed, the development sleeve or the developer holding member should also be driven at a higher speed correspondingly. However, an excessively high speed of the development sleeve can cause a fluidity-improving agent to drop out of toner particles or to be embedded into the toner particles owing to the temperature rise of the main body of the copying machine and friction with the developing agent. Such a deteriorated toner may not be charged suitably, resulting in a lower development efficiency, and is liable to cause the drop of image density when used for a long period of time. The insufficient toner charge lowers the toner transfer efficiency to decrease the density of the transferred image, or weakens the toner-confining force of the transferring electric field to cause scattering of toner particles and deterioration of image quality.
The on-demand fixing system intends energy saving. This system applies electric power only when the fixing is conducted for copying, without applying the power while the copying machine is stopping. In another fixing system, quick-start fixing is practicable in which the copying is conducted immediately after the turning-on of the copying machine without waiting time. In this system, fixing is conducted by heating and pressing by applying heat from a heater through a heat-conductive film to the toner on a recording medium instead of employing a heating roller (surf fixing).
In the surf fixing, however, owing to the low heat capacity of the film, the temperature of the portion of the delivered recording medium rushing to the film is lower than that of the portion of the film of heat-and-pressure fixing. Therefore, the toner particles in an nearly unmelted state on the recording medium rush to the film, which can bring about image defects of fixing scattering caused by a delicate air flow at the rushing portion of the recording medium to the film or by a electrostatic force acting between the toner particles and the film. This phenomenon is more remarkable in higher speed copying. This phenomenon of the fixing scattering can be prevented by development with a highly charged toner to form a toner image on a photosensitive member and transferring the toner image onto a recording medium to form an image in which toner particles are densely held.
The reuse of the toner recovered from the photosensitive member in the cleaning step is another problem arising in the system from the standpoint of environmental protection. After transfer of a developed toner image from a photosensitive member onto a recording medium, the toner remains partially on the photosensitive member. Conventionally, the remaining toner is recovered by a blade, a fur brush, a magnetic brush, or the like from the photosensitive member, and is stored in the main body of an image-forming apparatus. The recovered toner is finally discarded.
From the standpoint of environmental protection, copying machines are proposed which have a reuse system for reusing a remaining toner after image transfer for image development as a mixture with a fresh toner. However, the toner remaining after image transfer is inferior to the fresh toner in fluidity and chargeability, and can cause aggregate and charging failure to occur, resulting in image defects. A simple mixture of a remaining toner and a fresh toner can cause problems in image formation.
To solve technically the problems in the reuse system, Japanese Patent Application Laid-Open Nos. 2-157765, and 6-59501 (corresponding to EP-A573933) disclose control of particle size distribution of the toner to be used. Further improvement of the reuse system is demanded. For example, a high-speed copying machine (or a high-speed printer), which conducts a large number of copying operations such as copying of 60 or more A4-size recording paper sheets, recovers a large amount of unused toner from an electrostatic image holding member (e.g., photosensitive drum or photosensitive belt) in a cleaning step after image transfer in comparison with a low- or medium-speed copying machine. The recovered toner has a low fluidity, tending to form aggregate. Even with the proposed reuse system, the aggregatable recovered toner is not readily reusable without lowering the image quality in the high-speed copying machine in comparison with the reuse in the low- or medium-speed copying machine. In particular, a one-component magnetic toner as the developing agent is more difficult to reuse than a two-component developing agent composed of a nonmagnetic toner and a magnetic toner.
For the stabilization of the toner chargeability, various developing agent constitutions and development devices are disclosed. For example, Japanese Patent Application Laid-Open No. 9-26699 discloses an arrangement of a development sleeve and an auxiliary development sleeve close to a photosensitive drum to prevent a development ghost and toner deterioration. This is effective to some extent in preventing the development ghost and the toner deterioration. With this arrangement, however, a fine particulate toner having a large specific surface area may not readily be frictionally charged uniformly since the frictional charge is applied to the toner only by the development sleeve and a control blade. Further, for the formation of images having various image ratios with uniformly high image density, a member is necessary in which a toner is uniformly fed in the lengthwise direction of the development sleeve in a development device.
In a development device of a toner-replenishing type, differently from a cartridge type one used in LBP, the toner held in the device, the replenished toner, and the recovered toner are different from each other in fluidity and chargeability, and therefore the respective toners should be mixed sufficiently by stirring before use for the development. The toner mixed insufficiently, when applied onto a development sleeve, has a broad charge distribution, and may produce toner particles charged in opposite polarity. The oppositely charged toner particles are liable to adhere to the white blank portion of the image to cause reversed fogging.